Earth-boring tools for forming wellbores in subterranean earth formations may include a plurality of cutting elements secured to a body. For example, fixed-cutter earth-boring rotary drill bits (also referred to as “drag bits”) include a plurality of cutting elements fixedly attached to a bit body of the drill bit. Similarly, roller cone earth-boring rotary drill bits include cones mounted on bearing pins extending from legs of a bit body such that each cone is capable of rotating about the bearing pin on which the cone is mounted. A plurality of cutting elements may be mounted to each cone of the drill bit.
The cutting elements used in such earth-boring tools often include polycrystalline diamond cutters (often referred to as “PDCs”), which are cutting elements that include a polycrystalline diamond (PCD) material. Such polycrystalline diamond cutting elements are formed by sintering and bonding together relatively small diamond grains or crystals under conditions of high temperature and high pressure in the presence of a catalyst (such as cobalt, iron, nickel, or alloys and mixtures thereof) to form a layer of polycrystalline diamond material on a cutting element substrate. These processes are often referred to as “high-pressure, high-temperature” (or “HPHT”) processes. The cutting element substrate may be a cermet material (i.e., a ceramic-metal composite material) such as cobalt-cemented tungsten carbide. In such instances, the cobalt or other catalyst material in the cutting element substrate may be drawn into the diamond grains or crystals during sintering and serve as a catalyst material for forming a diamond table from the diamond grains or crystals. In other methods, powdered catalyst material may be mixed with the diamond grains or crystals prior to sintering the grains or crystals together in an HPHT process.
Cobalt, which is commonly used in sintering processes to form PCD material, melts at about 1,495° C. The melting temperature may be reduced by alloying cobalt with carbon or another element, so HPHT sintering of cobalt-containing bodies may be performed at temperatures above about 1,450° C.
Upon formation of a diamond table using an HPHT process, catalyst material may remain in interstitial spaces between the grains or crystals of diamond in the resulting polycrystalline diamond table. The presence of the catalyst material in the diamond table may contribute to thermal damage in the diamond table when the cutting element is heated during use, which heating is caused by friction at the contact point between the cutting element and the formation. Polycrystalline diamond cutting elements in which the catalyst material remains in the diamond table are generally thermally stable up to temperatures of about 750° C., although internal stress within the polycrystalline diamond table may begin to develop at temperatures exceeding about 350° C. This internal stress is at least partially due to differences in the rates of thermal expansion between the diamond table and the cutting element substrate to which it is bonded. For example, diamond has a linear thermal expansion coefficient (TEC) at 25° C. of about 0.8·10−6K−1, whereas cobalt has a TEC at 25° C. of about 12·10−6K−1. At 800° C., diamond has a TEC of about 4.5·10−6K−1, and cobalt has a TEC of about 17.0·10−6K−1. At temperatures of about 750° C. and above, stresses within the diamond table may increase significantly due to differences in the coefficients of thermal expansion of the diamond material and the catalyst material within the diamond table itself. For example, cracks may form and propagate within a diamond table including cobalt, eventually leading to deterioration of the diamond table and ineffectiveness of the cutting element. Besides being a source of thermomechanically initiated stresses, catalyst materials used to form polycrystalline diamond can also catalyze the phase transformation of diamond into graphite (commonly referred to as “reverse graphitization”), which contributes to degradation of diamond tables.
To reduce the problems associated with catalyst material (e.g., different rates of thermal expansion in polycrystalline-diamond cutting elements and reverse graphitization), so-called “thermally stable” polycrystalline diamond cutting elements have been developed. Such a thermally stable polycrystalline-diamond cutting element may be formed by leaching the catalyst material (e.g., cobalt) out from interstitial spaces between the diamond grains in the diamond table using, for example, an acid. All of the catalyst material may be removed from the diamond table, or only a portion may be removed. Thermally stable polycrystalline diamond cutting elements in which substantially all catalyst material has been leached from the diamond table have been reported to be thermally stable up to temperatures of about 1,200° C. It has also been reported, however, that fully leached diamond tables are relatively more brittle and vulnerable to shear, compressive, and tensile stresses than are non-leached diamond tables. In an effort to provide cutting elements having diamond tables that are more thermally stable relative to non-leached diamond tables, but that are also relatively less brittle and vulnerable to shear, compressive, and tensile stresses relative to fully leached diamond tables, cutting elements have been provided that include a diamond table in which only a portion of the catalyst material has been leached from the diamond table.